Add information about supported <value> to CSP header page

[contentfree]

Request type

• [ ] Please close this issue, I accidentally submitted it without adding any details
• [ ] New documentation
• [x] Correction or update

Details

At the CSP header page, the directives are clearly explained but nothing is said about acceptable values. You can glean some info from the examples such as being able to specify just "https:" to only allow HTTPS requests or "none". But publishing detailed information would surely be helpful.

[MendyBerger]

Would like to take this one

[chrisdavidmills]

@MendyBerger assigned; thanks!